Some interesting techniques.
I would just do it as a sweep. Select the top edge of each of the long sides going down the hill, including the curved parts all the way to the bottom. Copy-paste to create lines and Join each if necessary, then choose the narrow top edge and Sweep with the two downhill lengths as rails. As long as that bottom edge is a straight line it will work great. If there were curving on the bottom edge then I would just extend the point of the rails fully past the curve and after Sweeping use the curve bottom edge to Trim the surface.